1. Field of the Invention
This invention relates to a zoom lens, and particularly to a great aperture zoom lens in which a maximum angle of view includes a wide angle of view exceeding 60.degree. and which has a relatively wide magnification change range from the wide angle to the telephoto and is as bright as the order of F2.8.
2. Related Background Art
For example, U.S. Pat. No. 3,584,935 is known as a zoom lens of which the zoom magnification change ratio is of the order of three times and which has a wide angle end at one end of the magnification change range and in which F number is of the order of F2.8 and which has realized a great relative aperture. In zoom lenses, focusing is generally done by the so-called fore lens axial movement system, but U.S. Pat. No. 5,144,488, U.S. Pat. No. 4,460,251, etc. describe zoom lenses in which other focusing systems are adopted.
The zoom lens proposed in the aforementioned U.S. Pat. No. 3,584,935 is a zoom lens of four-unit construction comprising positive, negative, positive and positive lens units which is basically similar to the present invention, and has a great relative aperture in which fully open F number is as bright as the order of F2.8. However, this conventional zoom lens cannot be said to be sufficient in aberration correction, and is insufficient particularly in the correction of the fluctuation of downward coma by magnification change and distortion, and it has a large fore lens diameter, which is not preferable.
Also, as a focusing system, a zoom lens of four-unit lens construction comprising positive, negative, positive and negative lens units in which focusing is effected by only the second lens unit is disclosed, for example, in U.S. Pat. No. 5,144,488. In this zoom lens, however, fully open F number is as dark as F4-5.6, and in such a zoom lens, it is difficult to realize a great aperture zoom lens of the order of F2.8. Further, in the focusing system by the movement of only the second lens unit, aberration fluctuation is great, and particularly the fluctuations of spherical aberration and curvature of image field on the telephoto side are great, and this is not preferable. Accordingly, it is difficult to use this focusing system in a zoom lens having a greater relative aperture of the order of F2.8, and in terms of performance as well, remarkable deterioration of the short distance performance particularly on the telephoto side is brought about, and this is not preferable.
U.S. Pat. No. 4,460,251 discloses a zoom lens in which a second lens unit, a third lens unit and a part of a fourth lens unit are moved as a unit to thereby effect focusing. This zoom lens, however, is a zoom lens of four-unit construction comprising positive, negative, negative and positive lens units and therefore, differs in basic construction from the zoom lens of four-unit construction comprising positive, negative, positive and positive lens units. In addition, in this zoom lens, the image circle is remarkably small and the back focal length is also remarkably small and moreover, many lens units need be moved for focusing. For example, it is necessary to separate the fourth lens unit into two portions and move one of the two portions during focusing and therefore, mechanical structure becomes complicated and thus, this zoom lens becomes substantially equivalent to a zoom lens of five-unit construction, and this is not preferable. Also, the second and third lens units and a part of the fourth lens are all moved as a unit for focusing and therefore, the fluctuations of aberrations such as coma and curvature of image field are great, and this has not been preferable.